Frequency-modulation demodulator system



Dec- 25, J A. WORCESTER, JR 7 FREQUENCY MODULATION DEMODULATOR SYSTEMFiled April 2, 19,48

TO SOURCE 0F m1. SIGNALS AUDIO TO SOURCE OF F.l"1. SIGNALS Inventor:Joseph A.Worc ester'-',Jr-.,

b mam His Attorney.

Patented Dec. 25, 1951 UNITED: STATES PATENT orrics FREQUENCY-MODULATIONDEMODULATOR SYSTEM Joseph. A. Worcester, Jr., Syracuse, N; Y., as-

signor to General. Electric Company, a corporation-of New YorkApplication April 2, 1948, Serial No. 18,645

A still further. object of'thisi'invention: is to provide a frequencymodulation receiving system in which the function: of limitinganddemodulating are performed in a 'single'tube, precluding the need forseparate limiter stages, frequency discriminator transformers and thelike.

Yet a further object of this invention is to provide an electron beam'tube for use in fre=- quency modulation receiving systems and the like:

wherein the output of the final intermediate frequency stage in suchasys'tem: may be coupled directly to'this tube; and audio-frequencypoten-- tialsmaybe recovered. from the output thereof-,. in response to,frequency modulated signals; and wherein the output is substantiallyzero for. am.- pli-tude modulated signals.

Still another object of; this'invention: is toiprovide a highlyefficient; and highly stable frequency modulation receiving system whichis relatively simple and economical to'construct. and in which thenecessary component parts and: circuits. are reducedto a minimum.

The features of this invention: which archelieved to be-new aresetforth- Withparticularity in the appended claims. The inventionitself; however, together with further objects and'advantages thereofmaybest be'understood by'refi erence to the. accompanying drawing, inwhich,v

Fig. 1 shows one. embodimentoii the invention,

Fig. 2 shows a portion of the system: of Fig. 1..

useful for describing the operation thereof, and Fig. 3 shows apreferredmodification of the invention, incorporating a beam centeringarrangement.

It ha long been desirable to evolve a system which combines thefrequency modulation functions of limiting and detecting in a singletube" in order to eliminate the limiter stage, the frequencydiscriminator transformer, detecting diodes, and the multiplicity ofsmall components.

(Cl. v25027) associated with conventional frequency modu lationreceiving systems. Such a system is shown in Fig. l.

Referring now to Fig. 1, an electron beam tube,

. constructed in accordance with the present invention, is representedgenerally as l. The present tube is similar fundamentally to aconventional cathode-ray tube, in that. focusing elec-- trodes 2, 3 and4, connected to suitable taps on a source of unidirectional potential 5,are used to focus a beam of electrons into a beam of extremely smallcross-sectional area. This beam of electrons is produced by a usualcathode 6 connected to the negative terminal of source 5 The tube l' andheated by heating elements I. also contain conventional horizontal andvertical deflecting electrodes 8, 8 and 9, 9 to deflect the beam, in amanner to be described. The difference between the present tube and theconventional cathode-ray'tube lies in that a special plateconstructionreplaces the cylindrical high potential anode andfluorescent screen of the usual tube. This plate construction includesmetallic circular disk quadrants l0 and I l which are connectedtogether, and to a source of unidirectional.

potential B+ through load resistor l2. Load resistor l'2' is'by-passedfor intermediate frequencies by integrating. capacitor 1 3, andconnected toterminals M through capacitor [5. The abovementioned plateconstruction also includes a furthercirculardisk-like plate It which islocated a shortdistance behind sections HI and It with respect to theelectron beam. The plate [6 is connected directly to unidirectionalsource 3+ and. acts as a collector-for the electron beam when. it is.deflected other than on the segments Miami. Is.

The final: intermediate frequency amplifier stageofthe frequencymodulation radio receiving system is shown generally as H. The stagesofthis system, preceding the final intermediate fre-- quency stage, areconventional and may take any known form, and-a detailed description ofthese stages is believed to be unnecessary. Stage IT includes anelectron device l8 connected in the usual manner. The'cathode of deviceI8" is connected to-ground through biasing resistor I9 by--- passed bycapacitor 2 0: Control'electrode 21 is coupled to the preceding stagethe usual manner, screen: electrode 22" is connected to unidirectionalsource B'+ through resistor 23 and to ground through a capacitor 24, andsuppressor electrode 25 is connected directly to" the cathode The anode26 is coupledtouniof device 18. directional source -B+- through theprimary-winding 21 of intermediate frequency transformer 28. the primarywinding 21 being tuned to the intermediate frequency of the system byvariable capacitor 29. The primary winding 21 is coupled to thehorizontal deflection plates 8 and 8 through capacitors 30 and 3!, andsecondary winding 32 of transformer 28, tuned to the'intermediatefrequency by capacitor 33, is' coupledto the vertical deflection plates9 and 9 through capacitors 34 and 35.

Referring now to Fig. 2, there is shown a view of the sector plates Iand II and plate l6 taken from the cathode end of the electron beam tubeof Fig. 1. At the resonant frequency of the intermediate frequencytransformer 28 of Fig.- 1, a 90' phase angle exists between the primaryand secondary potentials thereof, which causes the electron beam totrace a circular pattern such as the dotted figure l'2'3-4'. It can beseen that during the time interval of each revolution of the electronbeam, the beam contacts the circular plate 16 for one-half of this timeinterval and the sectors ID and H for the other half. When the frequencyof the received signal is deviated from the resonant frequency 'of thecircuit, the circular pattern changes to an ellipse tending towards theline l-3 for a frequency deviation in one direction, and towards theline 2'4' for a frequency deviation in the opposite direction. In theformer case the beam falls entirely on the collector plate H5, and inthe latter case entirely on the segments (8 and H, except for the smallgap at the center. 'It can then be seen that the average beam currentvaries substantially from zero at one extreme of frequency deviation ofa received signal to a-maximum at the other extreme of this frequencydeviation. The beam current has a value midway between zero and themaximum, when the frequency of the received signal corresponds to theresonant frequency of the intermediate frequency circuit. Hence,integrating capacitor [3 removes the intermediate frequency component,and an audio potential, varying in accordance with the frequencydeviation of the applied signal, may be derived across the terminals !4.

The present system is responsive solely to frequency modulated signals,and it can be shown that the system is insensitive to amplitudemodulated signals. Considering the case where an applied signal has afrequency corresponding to the resonant frequency of the intermediatefrequency transformer 28, a circular pattern is obtained on the electrontube plate construction.

It is evident that amplitude changes in the applied signal merely changethe diameter of the circular figure I'2'3--4 and hence do not effect theoutput of the device, as-the beam contacts the segments (8 and II forone-half of the total time interval, regardless of the size of thecircular figure. The same situation exists during the various off-tuneconditions, as merely the size of the pattern alone changes, and theshape remains the same and hence the integrating-current through loadresistor 12 is unchanged.

The coupling between the intermediate frequency transformer 28 and thedeflecting elec-- trodes 8 and 9 of Fig. 1 may be double-ended, as shownin that figure, or on the other hand, this connection may be of thesingle-ended variety with one horizontal and one vertical deflectingelectrode operating at zero intermediate frequency potential, such anarrangement being shown in Fig. 3.

Referring now to Fig. 3 in detail, there is shown the final intermediatefrequency stage I! of a frequency modulation receiving system similar toFig. 1. The output transformer 28 of this stage has one side of itsprimary winding 2! connected to a source of unidirectional potential andthe other side thereof coupled to the vertical deflection electrode 9-through capacitor (38. The secondary winding 32 of transformer-28 hasone side connected to ground and the other coupled to the horizontaldeflecting electrode 8 through capacitor 34. V

In other respects the circuit of Fig. 3 is similar to that of Fig. 1,with the exception that means are provided for accurately centering theelectron beam when no deflecting potential is impressed on thehorizontal and vertical deflecting electrodes. The centering of the beamis accomplished by dividing the collector plate into two semi-circularsegments I8 and I6 these segments being connected to the unidirectionalsource 3-}- through resistors 36 and 31. The potentialdrop appearingacross resistors 38 and 31 is applied to the horizontal deflectingelectrodes 8' and 8 respectively through resistors 38 and 39, inconjunction with the inherent circuit capacitance, preventingintermediate frequencies from being applied to the deflecting electrodes8 and 8'. Hence, when the beam is inaccurately centered and falls, forexample, on plate l6, current flows through resistor 31 causing apotential to be applied to deflecting electrode 8, which consequentlydeflects the beam in a compensating direction. Likewise, segments l0 andH are connected to the audio load resistor 12 through respectivecompensating resistors 48 and 4|, the potential appearing across theseresistors being applied respectively to the vertical deflectingelectrodes 9' and 9 through resistors 42 and 43. The combined action ofthese compensating potentials causes the electron beam to seek itscentral position. The above-described compensating action also preventsany tendencies of the beam to be deflected by extraneous hum potentialsand. the like.

The 'beam centering circuits per se are more particularly described andclaimed in the copending application of Robert B. Dome, Serial No.4,810, filed January 28, 1948, for Deflection Limiter and assigned tothe same assignee as the present invention.

This invention therefore provides an improved frequency modulationdemodulator which uses an electron beam tube of special construction,and when utilized in conjunction with a frequency .modulation receivingsystem, renders the need for amplitude limiter stages, frequencydiscriminator transformers and other associated components, unnecessary.

While certain specific embodiments have been shown and described, itwill of course be understood that various modifications may be madewithout departing from the invention. The appended claims are thereforeintended to cover any such modifications within the true spirit andscope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A frequency modulation demodulator system comprising a source ofsignals frequency modulated about a center carrier frequency, an elec--tron beam tube and an output circuit, said electron beam tube includinga source of electrons, beam focusing means, horizontal and vertical beamdeflecting means, and beam collecting means, said beam collecting meanscomprising a first pair of diametrically-opposed sector members, asecond pair of diametrically-disposed sector members spaced from saidfirst pair and not more than partially shielded thereby, means forimpressing said signals across said deflecting means with the phase or"said center frequency impressed on said vertical deflecting meansdisplaced by 90 from the center frequency impressed on said horizontalmeans, a first and second resistance means for coupling respectively theopposing sector members of said first and second pairs, means forcoupling said output circuit to said first and second resistance meansrespectively, means for deriving audio signals from said output circuitin response to said frequency modulated signals, means for impressing apositive voltage with respect to said electron source to intermediatepoints of said first and said second resistance means respectively, anddirect current connections for applying potentials developed across saidfirst resistance to said horizontal beam deflecting means and forapplying potentials developed across the other of said resistance meansto said vertical beam deflecting means to center said beam.

2. A frequency modulation demodulator system comprising a source ofsignals frequency modulated about a center carrier frequency, anelectron beam tube and an output circuit, said electron beam tubeincluding a source of electrons, beam focusing means, horizontal andvertical beam deflecting means, and beam collecting means, said beamcollecting means comprising a first pair of diametrically-opposed sectormembers, a second pair of diametrically-disposed sector members spacedfrom said first pair and not more than partially shielded thereby, meansfor impressing said signals across said deflecting meanswith the phaseof said center frequency posing sector members of said first and secondpairs, means for coupling said output circuit to intermediate points ofsaid first and second resistance means respectively, means for derivingaudio signals from said output circuit in response to said frequencymodulated signals, means for -impressing a positive voltage with respectto said electron source to said intermediate points on said first andsaid second resistance means respectively, and direct currentconnections for applying potentials developed across said firstresistance to said horizontal beam deflecting means and for applyingpotentials developed across the other of said resistance means to saidvertical beam deflecting means to center said beam.

JOSEPH A. WORCESTER, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Metcalf Sept. 19, 1950

